Gas-liquid-liquid reaction engineering: the Koch synthesis of pivalic acidfrom iso- and tert-butanol; Reaction kinetics and the effect of a dispersed second-liquid phase

In gas-liquid-liquid reaction systems with fast parallel and consecutive re actions the effects of mass transfer and mixing on the product yield can be significant. The Koch synthesis of pivalic acid, using sulfuric acid as ca talyst, was chosen to study these effects. Reaction kinetics and the effect of the catalyst-phase composition have been investigated by using isobutan ol as reactant. For studying the effect of an immiscible liquid phase on th e reaction products obtained, the more reactive tert-butanol was used. Piva lic acid can be produced from isobutanol using sulfuric acid as a catalyst solution with 2-methylbutanoic acid as main byproduct, if gas-liquid mass t ransfer limitations are excluded. The selectivity towards 2-methylbutanoic acid is generally less than 20% and decreases strongly with decreasing acid ity. The reaction is first order in isobutanol and dehydration is likely to be rate determining. The presence of pivalic acid and isobutanol strongly reduces the apparent reaction rate constant by decreasing the solution acid ity (Ho). For the industrially applied backmixed reactors in the Koch synth esis, this may imply that these operate at much lower values for Ho. On add ition of an immiscible heptane phase, the reaction products are extracted t o some extent and this adds to maintaining a high catalyst solution acidity . Using tert-butanol, the yield and pivalic acid selectivity was found to d epend strongly on CO transport to the reaction zone through gas-liquid mass transfer and mixing. The presence of an immiscible heptane phase increased the product yield and selectivity towards pivalic acid significantly. (C) 1999 Elsevier Science Ltd. All rights reserved.